In cellular wireless communication networks, or "cellular networks," a served area is divided into cells. Each cell is further divided into sectors, except in the case of omni-directional cells, in which the entire cell comprises a single sector. Each cell is served by at least one base station located at a cell site typically at the center of the cell. All of the base stations are connected to a message switching center ("MSC") via a base station controller ("BSC") and hardware links. A plurality of mobile units are connected to the MSC by establishing radio links with one or more nearby base stations.
In communication systems that utilize narrow-band modulations, such as analog frequency modulation ("FM"), the existence of multiple paths ("multipath") causes severe fading. However, with wideband CDMA modulation, the different paths may be independently received, thereby greatly reducing the consequences of the multipath fading. However, multipath fading cannot be completely eliminated due to the occasional occurrence of unresolved multipath, i.e., multipath that cannot be independently processed.
Diversity is the approach most commonly used to mitigate multipath fading. In a CDMA cellular network, or "CDMA network," three forms of diversity are used. These include:
time
symbol interleaving, error detection, and correction coding PA1 signal energy is spread over a large bandwidth PA1 1. multiple signal paths from simultaneous links between mobile station and different sectors (soft handoff) PA1 2. RAKE receivers are used to combat the multipath environment by separately combining signals arriving with different (resolvable) propagation delays PA1 3. multiple, typically two, antennas at each cell site, wherein all of the antennas at a single cell site are designed to the same specifications PA1 N is the number of users an average sector can support; PA1 f.sub.pilot is the fraction of total sector high power amplifier ("HPA") power allocated for the pilot channel; PA1 f.sub.page is the fraction of total sector HPA power allocated for the paging channel; PA1 f.sub.synch is the fraction of total sector HPA power allocated for the synch channel; PA1 f.sub.user.sub..sub.--avg is the average fraction of total sector HPA power allocated to a user; PA1 hrf is the handoff reduction factor, a calculated value that takes into account the required resources due to different types of handoff; and PA1 v is the average voice activity factor. PA1 N is the number of users per sector; PA1 W is the spread-spectrum bandwidth; PA1 R is the data rate; PA1 E.sub.b /N.sub.o is the ratio of energy per bit (E.sub.b) to the noise power spectral density (N.sub.o); PA1 v is the average voice duty cycle; and PA1 F is the frequency reuse factor.
frequency
space
One of the most important effects achieved by improving a CDMA network is the increase in the network's capacity; that is, the number of calls that can be handled by the network at a given time. It should be noted that the capacity of a CDMA network is soft, i.e., the capacity of the network can be increased, but with a corresponding decrease in call quality.
CDMA network capacity takes two forms, which are forward link capacity and reverse link capacity. In practical CDMA networks, forward link capacity is the limiting form of capacity. The forward link capacity of a CDMA system is dependant on handoff and forward link transmit power requirements between sectors and mobile stations. A higher handoff and higher transmit power requirement will compromise the CDMA capacity. The following equation relates the forward link capacity to the average forward traffic channel gain and average soft handoff percentage for a CDMA network: EQU N=(1-(f.sub.pilot +f.sub.page +f.sub.synch))/(f.sub.user.sub..sub.--avg *hrf* v) (1)
where:
It should be noted from equation (1) that if the factors hrf and f.sub.user--avg are reduced, the overall forward link capacity of the network will be increased.
The reverse link pole (i.e., maximum) capacity may be estimated using the following equation: EQU N=(W/R)*(1/(E.sub.b /N.sub.o))*(1/v)*F (2)
where:
Frequency reuse factor is the ratio of the interference from mobile units within a sector to the total interference from mobiles in all sectors. Wider antennas result in marginally lower frequency reuse factors.
The capacity of a network is typically increased via sectorization. This is accomplished by the use of directional antennas. A directional antenna reduces the interference seen at a base station because it only receives in the direction of the antenna. In fact, if the antenna had no side-lobes or back-lobes, which reduce the frequency reuse factor (F), the out-of-interference would be further reduced, increasing F.
Depending on the purpose of the particular CDMA network considered, cell site separation may be designed based on "link budget" calculations. The link budget enables the network planners to separate the cell sites as far as possible, while maintaining adequate coverage or coverage to a given grade of service. In such cases, the reverse link budget is used to determine cell site separation.
In other cases, in particular, when there is a surplus link budget, the cell sites are positioned in closer proximity to one another. In cases where the network is designed for capacity, there is a surplus link budget. In these cases, higher capacity means a greater number of users in a given area.
In FIG. 1, a typical CDMA network is designated generally by a reference numeral 10. In a preferred embodiment, the system 10 is comprised of a plurality of cells, represented in FIG. 1 by cells C1 and C2. Each of the cells C1, C2, is divided into a plurality of sectors S1, S2, S3 and S4, S5, S6, respectively, through use of a plurality of directional antennas (FIG. 2) located at or near a respective base station BS1, BS2. Although the cells C1, C2, are shown as being divided into three sectors, it will be recognized by those skilled in the art that cells may be subdivided into one or more sectors depending on the configuration of the system 10. As previously noted, each cell C1, C2, comprises a base station B1, B2, respectively, the primary function of which is to provide over-the-air radio frequency ("RF") communication with mobile units, such as a mobile unit 12.
Each base station B1, B2, is further connected via a link to a base station controller ("BSC") 18, which is connected to a mobile switching center ("MSC") 22. As the elements comprising the system 10, as well as the configuration thereof, are well known in the art, the details thereof will not be further described, except as necessary to impart a complete understanding of the present invention.
As illustrated in FIG. 2, for each sector, there are typically two antennas 200, 202, located at a "sector site" 204 thereof. In some cases, as shown in FIG. 2, one of the antennas 200 functions as a hybrid transmit/receive antenna, while the other antenna 202 functions as a receive only antenna. The antennas 200, 202, are usually are separated far enough apart to ensure that the individual energy that each antenna captures has faded independently. Generally, both antennas have been of the same specifications, including antenna beamwidth, material, size, gain, and others.
Clearly, increasing the capacity and reliability of a CDMA network is a constant focus of network planners. Therefore, what is needed is a technique for improving both link performance (i.e., coverage) and capacity of a sectorized CDMA network.